A multi-network SIM is a SIM that can connect to more than one mobile network in a country, so a device can move to another network when the one it is using becomes unavailable. In the UK, a multi-network SIM is often called a UK roaming SIM, and can use EE, O2, Vodafone and Three.

A multi-network SIM does not choose the network itself. The device, usually a router or IoT module, chooses which network to register on from the networks the SIM is allowed to use. Once registered, the device normally stays on that network until it loses service. It does not keep switching to whichever network has the strongest signal at that moment.

When the device loses service, it searches for another permitted network and registers on it. The switch is not instant, because the device first has to detect that the connection has failed. Many industrial routers can monitor the connection and force a reconnection when traffic stops, which shortens the time the device is offline.

A multi-network SIM can be steered or unsteered. A steered SIM is pushed by its provider onto a preferred network, usually for cost reasons, even where another network would work better. An unsteered SIM lets the device use any of the permitted networks. Millbeck UK roaming SIMs are unsteered.

Most multi-network SIMs work by roaming: the SIM belongs to one operator, which has agreements to use several local networks. Some use multi-IMSI, where the SIM holds more than one network identity and switches between them. Either way, a multi-network SIM protects against coverage gaps and outages on one network, but it does not fix a poor installation, such as a router with its antennas inside a metal cabinet. See UK Roaming SIMs for how Millbeck supplies them.